In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine section mounted on the same shaft. In the turbine section, the hot combustion-gas flow passes between pairs of turbine vanes (also sometimes termed the “nozzles”), which redirect the combustion-gas flow slightly, and impinges upon the turbine blades. The impingement of the flow of hot combustion gas against an airfoil section of the turbine blades turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. However, at these high temperatures the combustion-gas flow is highly corrosive, erosive, and oxidative to the materials it contacts. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine components of the engine, such as the turbine blades and vanes. The turbine components must have the necessary strength, but also be resistant to the environmental damage caused by the combustion-gas flow, at the operating temperature. In current engines, the turbine vanes and blades are made of cobalt alloys and nickel-based superalloys, and can operate at temperatures of up to about 1800-2100° F. These components are subject to environmental damage by corrosion, erosion, and oxidation at those temperatures.
Many approaches have been used to increase the operating temperature limits and service lives of these hot-section turbine components to their current levels, while achieving acceptable environmental resistance. The composition and processing of the base materials themselves have been improved. Cooling techniques are used, as for example by providing the turbine component with internal cooling passages through which cooling air is flowed.
In another approach used to protect the turbine-section components, a portion of the surfaces of the components are coated with a protective coating. One type of protective coating includes an aluminum-containing protective coating deposited upon the substrate material to be protected. The exposed surface of the aluminum-containing protective coating oxidizes to produce an aluminum oxide protective scale that protects the underlying surface. A ceramic thermal barrier coating may be applied over the aluminum-containing protective coating to further protect and insulate the substrate.
Despite careful selection of the base materials and protective coating, after a gas turbine component has been in service, at least some parts of its are usually eroded, corroded, and oxidized so that one or more key dimensions of the component may be reduced below respective minimum permissible dimensional values. An example relates to the throat separation dimension between each adjacent pair of gas turbine vanes, the space through which the hot combustion-gas flows from the combustor on its way to contact the turbine blades. The throat dimension between two adjacent gas turbine vanes has its maximum permissible dimensional value that cannot be exceeded without reducing the performance and efficiency of the gas turbine. When the gas turbine vanes are operated in service, their surfaces are worn away so that a key thickness dimension of the vanes become dimensionally smaller. Consequently, the vane-to-vane gas-flow throat-separation dimension becomes larger and the maximum permissible throat-separation dimensional value is eventually exceeded as the key thickness dimension of the vanes falls below its minimum permitted key thickness dimension.
The gas turbine components are expensive to fabricate, and it is therefore desirable, where feasible and the service damage is not too great, to repair and restore them, rather than to discard them. No restoration procedure has been proposed for these protected components to restore dimensions and also to restore the protective structure, and therefore a need exists for such a restoration procedure. The present invention fulfills this need, and further provides related advantages.